In under-balanced drilling, as opposed to conventional drilling, down-hole well pressure at the formation is maintained below the formation pressure by the utilization of a relatively light base drilling fluid. The under-balanced condition avoids contamination of the formation by reducing the chances that the drilling fluids and the "cuttings," suspended solids produced by the action of the drill bit, will be forced into the permeable reservoir formation. Several types of base drilling fluid may be used in under-balanced drilling. Water-based and oil-based drilling muds may be used, however, water and lighter oil-based fluids, such as diesel fuel and crude oil, are more commonly used. In some situations the base drilling fluid will have a specific gravity too high to successfully use in an under-balanced well. In such situations, the controlled mixture of additives, such as nitrogen gas, to the base drilling fluid produces an operating fluid of a specific gravity selected to maintain an under-balanced well.
The higher formation pressures usually result in well formation fluids, such as hydrocarbon oil, hydrocarbon gas and well water, flowing into the well and mixing with the operating fluid and cuttings. The returning drilling stream reaches the surface wellhead as a mixture of formation oil, formation gas, well water, solid cuttings and operating fluid. If the operating fluid is oil-based, any liquid hydrocarbons produced from the well will mix with the operating fluid. Similarly, if the operating fluid is water-based, any well water produced will mix with the water-based operating fluid. If additive gasses were mixed in forming the operating fluid, the additive gases will mix with any hydrocarbon gas produced in the well.
In under-balanced drilling the returning drilling stream is at elevated pressures and when separation of the stream elements is desired, separation must be performed in a closed container or tank. Unfortunately, liquid-gas separation is less efficient when performed at elevated pressure levels. It is important to remove as much hydrocarbon gas from the base drilling fluids as possible. Basic hydrocarbon equilibrium phase behavior dictates that lowering the separation pressure reduces the hydrocarbon gas remaining in solution as a liquid. However, reducing the separation pressure, to release the gas from the liquid, increases the actual gas volume, thereby complicating gas handling and flow issues. In conventional, balanced drilling the operating fluid is not impregnated with large quantities of well formation fluids and, consequently, the operating fluid does not need to be separated from the returning stream at the surface at elevated pressures.
In a closed, balanced drilling system, controlling the specific gravity of the operating fluid flowing into the well is relatively uncomplicated, making maintenance of the stability of the well relatively simple. In under-balanced drilling the fluid mixture circulating in the well is not a closed system because of the addition of formation fluids down-hole. The influx of these formation fluids and gases greatly complicates the problem of under-balance pressure control through operating fluid specific gravity management.
Separation of the well formation fluids from the base drilling fluids is necessary before the base drilling fluids may be returned to the well and is accomplished by processing the returning stream through a separation system. The separation system should have the capacity to remove approximately the same or in excess of the volume of gas from the returning stream as is being added to the operating fluids down-hole. That is, the separation system should keep up with production of formation gas from the well to maintain the stability of the well during drilling operations.
Complicating matters, the separation system must handle typical wellhead pressures of the returning stream, which during under-balanced drilling can range from 25 psi to 3000 psi. Wellhead pressures are typically maintained as low as possible but still high enough to handle the returning stream volume. Reducing the pressure of the returning stream from the wellhead operating pressure by venting into a closed chamber can cause foaming, which reduces the efficiency of the liquid-gas separation process.
During drilling a large volume of heavy cuttings is produced and returned to the surface wellhead in the returning stream. In conventional drilling the returning stream is treated with shale shakers and mud pits. In under-balanced drilling it is necessary to remove the cuttings, or solids produced during drilling, from the returning stream mixture in the pressurized tanks to prevent clogging of the tanks. For safety reasons, in under-balanced drilling, it is first necessary to remove the gases from the returning stream. Removal of the solids from the pressurized chambers without shutting down the drilling operation presents difficulties.